The present invention is concerned with blood reservoirs, and in particular venous and cardiotomy blood reservoirs.
Many surgical procedures require that the patient's blood be diverted outside the body. For example, during open heart surgery the patient's blood must be directed around the heart and lungs. This usually involves the set up of an extracorporeal circuit. Extracorporeal circuits generally include devices for performing various tasks on the blood, e.g. oxygenation, filtration, and storage. Other procedures requiring the routing of blood through an extracorporeal circuit include extracorporeal membrane oxygenation (long term support) and autotransfusion.
Extracorporeal circuits are typically set up by an individual known as a perfusionist. The perfusionist controls the rate of blood flow and operates the various devices connected in the circuit. Extracorporeal circuits generally include oxygenators, heat exchangers, and filters, which are interconnected by surgical tubing. These circuits also include reservoirs. A blood reservoir is an enclosure into which blood is temporarily stored. The storage of blood in reservoirs allows regulation of the patient's blood volume and pressure. Typically, reservoirs also include various elements to filter and defoam the blood.
Blood flowing through an extracorporeal circuit, and particularly through the oxygenator and filters, may entrap air in the form of fine bubbles. Any gas bubbles must be removed from the blood prior to reintroduction into the patient.
Reservoirs are generally of two types. A first type of reservoir, known as a closed reservoir, is one which is formed from a generally flexible bag or container. Blood will expand this type of reservoir as it enters the reservoir. The air to blood interface is limited by the lack of empty space in the reservoir prior to filling with blood. Generally such reservoirs are sealed from the external environment. Closed reservoirs have numerous advantages. Such reservoirs isolate the blood from air thus limiting the extent of the blood to air interface which is detrimental to blood components. Flexible containers collapse and expand as the quantity of blood varies without delivering large quantities of air downstream into the extracorporeal circuit. This reduces potential injury to the patient from air embolism, particularly when all of the blood is removed from the reservoir.
The use of a flexible shell is also a benefit with oxygenators. For example, see U.S. Pat. No. 3,545,937, issued to Rozhold et al on Dec. 8, 1970; U.S. Pat. No. 3,827,860, issued to Burlis on Aug. 6, 1974; U.S. Pat. No. 3,853,479, issued to Talonn et al on Dec. 10, 1974; U.S. Pat. No. 3,892,534, issued to Leonard on July 1, 1975; U.S. Pat. No. 3,915,650, issued to Talonn et al on Oct. 28, 1975; and U.S. Pat. No. 3,918,912, issued to Talonn et al on Nov. 11, 1975.
One disadvantage with closed reservoirs is the inability to separate gross amounts of incoming air. The separation of small amounts of air from the blood is also difficult with closed reservoirs absent screen filters. It is also difficult to remove any entrapped air from the reservoir without physically compressing the bag, or by sucking or pumping the air out of the bag with a syringe or similar device.
A second type of reservoir is known as an open reservoir, and is formed from a rigid or hardshell container. The reservoir is filled with air which is pushed out by the entering blood. Usually a large portion of the air is removed during the priming process, but a small volume of the reservoir remains filled with air during the operation of the reservoir. This provides for an air to blood interface, which as stated may lead to the damage of various blood components.
One major advantage with open reservoirs is the establishment of the air to blood interface. Any air present in the incoming blood normally rises upwards through the blood passing across the blood to air interface. Such reservoirs typically include filters and defoaming elements which further enhance the release of entrapped gas across the blood to air interface. The released air is vented to atmosphere. Unlike closed reservoirs, open reservoirs allow for a precise measurement of the blood volume. That is, unlike the expanding and contracting closed reservoirs, open hardshell reservoirs allow for visual inspection of the quantity of blood flowing through the reservoir. By providing the hardshell reservoir with visually readable volume markings the precise amount of blood can be ascertained during the surgical procedure.
Open reservoirs also tend to impose a lower back pressure on the extracorporeal circuit. That is, blood flow through the reservoir will not increase the back pressure in the upstream portion of the circuit. Closed reservoirs induce a greater back pressure in the upstream portion of the circuit which may increase the amount of blood forced into the heart.
As stated, entrapped gas bubbles will rise in the reservoir and pass across the air to blood interface. Even though less efficient than open reservoirs, some entrapped air passes across this interface in closed reservoirs. To facilitate the removal of this gas, reservoirs have been designed with air vents. The air escapes to the environment through these vents. An example of a blood reservoir having an air vent is disclosed in U.S. Pat. No. 4,643,713, issued to Viitala on Feb. 7, 1987.
Examples of flexible shell reservoirs are disclosed in many of the above patent references, while examples of hardshell reservoirs are found in some commercially available oxygenators, such as the BCM-7, a product manufactured and sold by the Baxter Healthcare Corporation, Deerfield, Ill., Capiox E. an oxygenator sold by Terumo Corporation, Tokyo, Japan; and the CML, an oxygenator sold by Cobe Corporation, Boulder, Colo.
Blood passing through an extracorporeal circuit will usually entrap smaller air bubbles which normally will not pass through the air to blood interface. Some reservoirs have been designed to promote the breakdown of these smaller gas bubbles by incorporating screens in the blood pathway. For examples of such reservoirs see U.S. Pat. No. 4,493.705, issued to Gordon et al on Jan. 15, 1985, and U.S. Pat. No. 4,734,269, issued to Clarke et al on Mar. 29, 1988. These reservoirs include 100 to 250 microns and 50 to 300 microns, respectively.
Screens have also been positioned in filtering bags. These bags are usually positioned at the upstream end of extracorporeal circuits, and filter out denatured blood components. Entrapped gas bubbles would be broken down passing through this filter bag assembly. An example of such a bag is disclosed in U.S. Pat. No. 4,035,304, issued to Watanabe on July 12, 1977.
It is also known that during the oxygenation process, particularly with bubble oxygenators, the mixing of gas and blood forms foam. Foam is highly undesirable. While the foam may be merely removed from the circuit, it is the usual practice to separate any blood from the foam first. This is usually accomplished by passing the foam through a porous element which is at least partially coated with a defoaming substance, such as silicone antifoam.
Reservoirs have been designed to include fiber or equivalent elements which are coated with this antifoam material. Silicone antifoam or a derivative compound, breaks foam down into blood and gas. The gas is usually vented to the environment. An example of such a reservoir is seen in U.S. Pat. No. 4,466,888, issued to Verkaart on Aug. 21, 1984.
One disadvantage with the use of silicone antifoam, or equivalent substance, is the detrimental effect on blood. This material can become dislodged and shed into the blood. The shed antifoam material can become lodged in the patient's vascular system disrupting blood flow. Some researchers have suggested a mechanism for limiting the potential exposure of blood with silicone antifoam. Specifically, a reservoir, in combination with an oxygenator, is disclosed in U.S. patent application Ser. No. 338,347 filed Apr. 12, 1989, which is a continuation of U.S. Ser. No. 885,963, and assigned to the same assignee as the instant application. This application also discloses a reservoir having a defoaming material generally positioned above the maximum blood level. As stated in this application, the positioning of the defoaming material above this maximum blood level reduces the potential contact between the blood and the material.
It is thus apparent that both flexible and hard shell reservoirs provide distinct advantages, but also possess separate and distinct disadvantages.